The present invention relates to a method and apparatus for a frequency hopping-spread spectrum wireless communication system, and, more particularly, to a method and apparatus for a frequency hopping-spread spectrum communication system suitably applied to CDMA (Code Division Multiple Access) communication between communication stations having forward-links and reverse-links with different wireless link qualities each other.
Spread modulation systems in a spread spectrum communication system include a direct sequence (DS) system which spreads the spectrum over a wide band by directly multiplying pseudo-random noise (PN) series on information modulated signals, and a frequency hopping (FH) system which uniformly spreads the spectrum by driving a frequency synthesizer in correspondence to hopping series to change (hop) the transmit frequencies of information modulated signals. The FH method in particular has an advantage that transmit power is not required to be controlled because there is no near-far problem, and that it is insusceptible to the frequency selective fading because there is a frequency diversity effect.
In addition, in a mobile communication, communication is performed between a base station and a mobile station by using a forward-link and a reverse-link for transmission and receiving. For its multiplexing, the spectrum spread communication system with the FH system is generally employed to particularly separate and multiplex frequency bands on the forward-link and the reverse-link.
For example, as shown in the time-frequency matrix of FIG. 1, eight hopping frequencies in total (f.sub.0, f.sub.1, f.sub.2, f.sub.3, f.sub.4, f.sub.5, f.sub.6, f.sub.7) are assigned to the forward- and reverse-links. In this case, the frequency bands on the forward- and reverse-links are separated into a high frequency band and a low frequency band, and multiplexed by using four hopping frequencies (f.sub.4, f.sub.5, f.sub.6, f.sub.7) for the reverse-link and four hopping frequencies (f.sub.0, f.sub.1, f.sub.2, f.sub.3) for the forward-link. Particularly, in FIG. 1, the hopping frequencies are used between the base station and one mobile station in temporal sequence of (f.sub.3, f.sub.1, f.sub.2, f.sub.0, f.sub.3) for the forward-link and (f.sub.5, f.sub.6, f.sub.4, f.sub.7, f.sub.5) for the reverse-link.
Moreover, various other systems are proposed for setting frequency bands for the forward- and reverse-links. For example, a system described in Japanese Patent Application Laid-Open No. 8-181680 is shown in FIG. 2. This system performs transmission and receiving by using a same frequency band of (f.sub.0, f.sub.1, f.sub.2, f.sub.3, f.sub.4, f.sub.5, f.sub.6, f.sub.7) for the forward- and reverse-links, and by multiplexing using different hopping frequencies such as a frequency band of (f.sub.1, f.sub.3, f.sub.5, f.sub.7) for the forward-link and a frequency band of (f.sub.0, f.sub.2, f.sub.4, f.sub.6) for the reverse-link. Particularly, in FIG. 2, the hopping frequencies are used in a temporal sequence of (f.sub.7, f.sub.3, f.sub.5, f.sub.1, f.sub.7) for the forward-link and (f.sub.2, f.sub.4, f.sub.0, f.sub.6, f.sub.2). With such method, since the hopping frequencies for the forward- and reverse-links are mixed, the frequency diversity effect can be expected even if a frequency band over a wide band is affected by frequency selective fading or the like.
In addition, Japanese Patent Application Laid-Open No. 6-104865 describes a CDMA system in which transmission power control with high accuracy even against fading is attained by using same carrier wave frequencies for the forward- and reverse-links, and performing communication with time division multiplexing. A technique is also disclosed, when link capacity is different for the forward-link and the reverse-link, to accommodate such difference by changing the transmission time of the same carrier wave frequencies.
The conventional frequency hopping-spread spectrum communication system has advantages that the transmission power control imposes less restriction, and there is the frequency diversity effect. However, there is a possibility that there arises difference in link quality for the forward- and reverse links by, for example, difference in the size of antennae between the base station and the mobile station. That is, while the base station uses a large antenna providing sufficient gain, the mobile station is forced to use a small antenna because of necessity for smaller device size. Thus, the reverse-link from the base station to the mobile station is susceptible to radio propagation, so that the bit error rate (BER) of the receive signal tends to be deteriorated. On the other hand, BER is less deteriorated from the mobile station to the base station. The bit error rate (BER) is differently deteriorated by the frequency selective fading or the like due to difference of the frequency band used by the forward-link and the reverse-link.
The conventional system has a disadvantage that it cannot sufficiently accommodate such difference of line quality on the forward- and reverse-links. Particularly, when it is intended to attain the CDMA communication system in the mobile communication or the like, the frequency selective fading causes a considerable problem. The frequency hopping system is said to be inherently insusceptible to the frequency selective fading because it provides the frequency diversity effect. However, in the above-mentioned system to separate and multiplex the frequency bands for the forward- and reverse links, there is such a problem that, if either one of the frequency bands is affected by the frequency selective fading or the like, reliability of the affected link is lowered.
When the code division multiple access is applied to a cellular system, inter-cell interference on the forward-link is an average of interference from the mobile stations uniformly distributed in adjacent cells. However, the inter-cell interference on the reverse-link is required to be evaluated for a case where the mobile station subject to interference is at the worst location such as one end of a cell. Thus, the number of channels available for communication on the reverse-link is less than those on the up-link. Since the number of channels should be generally equal to the forward- and reverse-links, the upper limit of the number of channels is restricted by the number of channels for the reverse-link.
Furthermore, the technique disclosed in Japanese Patent Application Laid-Open No. 6-104865 is intended to attain transmission power control with high accuracy, so that the device becomes complicated because it performs time division multiplexing (TDM) in addition to the code division multiplexing.
The present invention is invented in view of the above problems, and intended to provide a method and apparatus for a frequency hopping-spread spectrum communication system which enables it to improves a link with poor link quality, and to equalize link quality by adequately distributing the number of hopping frequencies and the number of hops per data symbol in a transmission system with different link quality for a forward-link and a reverse-link.
In addition, the present invention is also intended to provide a method and apparatus for a frequency hopping-spread spectrum communication system which, in a transmission system with different link quality for an forward-link and a reverse-link, enables it to perform reliable communication with high frequency diversity effect even if it is affected by frequency selective fading by making the number of hopping frequencies being assigned different from the number of hops, and to equalize link capacity by taking deterioration of wireless link into account.